This invention relates to a brushless direct current motor, and more particularly to a brushless direct current motor designed for use with rotating disk systems. Even more particularly, the invention relates to a brushless direct current motor wherein all of the rotating components can all be balanced as a unit in one balancing operation, and wherein a static seal is used to prevent an air flow from entraining the lubricant from the bearings.
The use of brushless direct current motors in electronic equipment with rotating components, such as a magnetic disk storage system, has become common. There are two reasons for this: (1) the speed of rotation of such devices must be precisely controlled, and the brushless direct current motor, with its electronic control system, allows accurate speed control to be easily accomplished; and (2) the brushless direct current motor can be made an integral part of the mechanical support structure, thereby saving both space and cost.
In some uses of the brushless direct current motor, the rotating components of the motor must be balanced to an acceptable level. If the correct level of balance is not achieved, the electronic equipment using the motor may not function as desired, or the life of the motor may be shortened because of the stresses placed on the bearings from the wobble introduced by the imbalance.
Where the brushless direct current motor is used to drive the spindle of a disk storage system, the rotating disk pumps air and effectively lowers the air pressure (creates a vacuum) at one end of the spindle. Accordingly, unless some sort of seal is used to stop this air from being pumped into the motor housing, the air stream will entrain lubrication from the bearings. This lubrication then becomes a contaminant for the disk storage system, plus the bearings are dried out and subject to premature wear. Typically, some sort of dynamic seal is used to prevent such an air flow. For example, labyrinth, controlled gap, elastomeric mechanical contact or ferro-fluidic seals are all dynamic seals that are known in the art for use with rotating shafts. Unfortunately, these dynamic seals are costly and not always effective.
Balancing of brushless direct current motors is accomplished in the prior art by individually balancing each rotating component of the motor. The components are then assembled and the assembled unit is checked for the correct balance. Quite often, additional balancing is required.
If the motor should ever have to be disassembled, e.g., because of maintenance of the motor or the overall system in which the motor is used, additional balancing may be required when the motor is reassembled. This is because the individual components of the motor will probably not reassemble exactly as they were before disassembly. Two approaches have been used in the prior art to avoid this additional balancing step: (1) the individual motor components may be fabricated to very tight tolerances, thereby ensuring that proper balance will be obtained regardless of how the components are reassembled; or (2) keying pins and matching alignment holes may be selectively placed in the components, thereby maintaining a fixed relationship between the components each time reassembly occurs. While the use of tight tolerances and keying pins and alignment holes helps ensure that a motor can be reassembled without needing rebalancing, these approaches also disadvantageously add significantly to the cost of the motor. What is needed therefore is a low cost, simple brushless direct current motor design wherein maintaining the desired balance of the rotating components is no longer a problem, and wherein undesirable air flow into the bearings can be prevented.